Fertilization or Penetration of Ovum

Aug 13 2007

The essential step in the initiation of a new life is fertilization, the penetration of the ovum by a spermatozoon and the fusion of a portion of the two cells into a new single cell. From this united parent cell originate all the billions of cells that form a new being.

Nucleus

The part of each cell that fuses is called the nucleus. On a properly prepared microscopic slide it is the part of the cell that stains dark with aniline dyes. The nucleus is not a solid mass of tissue, as it may appear under low magnification, but is made up of a network of little rods called chromosomes.

Chromosomes

There is a specific number of chromosomes characteristic for each species, and every cell in the body of each animal belonging to that particular family contains this number of chromosomes. The famous fruit fly, which has contributed so selflessly to our knowledge of genetics, possesses only four pairs of chromosomes. In humans there are forty-six single chromosomes, but each has a counterpart; thus there are twentythree different pairs. Before fertilization is accomplished, the chromosome number of each human parent cell-sperm and ovum-has been halved from forty-six to twenty-three, one member of each of the twentythree pairs remaining in the fully mature sex cell. Thus when the two mature human sex cells fuse, each brings twenty-three chromosomes to the process of fertilization, and their union restores the human species number of forty-six chromosomes. Research has shown that these minute rods are the all-powerful agents in the transmission of hereditary characteristics. To them, each of us owes not only our sex, but our body build, coloring, and, in large measure, our mentality, emotional makeup, and longevity.

Genes

Chromosomes, in turn, are made up of chains of smaller genetic units called genes. The total number of genes in our twenty-three chromosomes is estimated at between fifty thousand and one hundred thousand. Since the individual genes are the ultimate determinants of genetic inheritance, the almost infinite variety of combinations explains why all humans differ so markedly, unless they are identical, one-egg twins. Oneegg twins have exactly the same chromosomal-gene makeup, since the egg divides after fertilization and from each half an independent embryo develops. Thus a pair of one-egg twin children contain exactly the same chromosomes and genes.